The present invention relates to a corrective lens, in particular to a spectacle lens having a large front warpage angle.
So-called wrap-around spectacles configured to have a relatively large front warpage angle to protect eyes of a wearer from light, wind, particles, etc., which would enter the eyes from temporal region sides of a head, have become widespread. However, most of the conventional wrap-around spectacles are configured as a non-corrective lens (i.e., glasses having no dioptric power). There are demands to use wrap-around spectacles which can provide proper visual acuity to a person who needs correction of visual acuity.
A spherical lens having a deep base curve (approximately 8 dioptre) formed to fit into a spectacle frame having a large front warpage angle such as wrap-around spectacles has been proposed. However, when such a spherical lens is worn by a wearer, even a visual axis defined when the wearer faces the front can not avoid intersecting with a lens surface obliquely. Therefore, large aberrations are caused when the spherical lens is used to correct visual acuity. Providing a negative power to a spherical lens having a deep base curve can lead to large aberrations even if the spectacle frame to which the spherical lens is to be fitted has no front warpage angle. That is, to design a spherical lens for wrap-around spectacles to attain refractive correction yields problems to be addressed.
In Japanese Patent Provisional Publication No. HEI 5-323185A (hereafter, referred to as JP HEI 5-323185A), a spectacle lens configured to have an extended toric surface on its back surface to attain refractive correction is disclosed. The extended toric surface is configured to have a symmetric property with respect to each of two axes (X-axis and Y-axis) perpendicularly intersecting with respect to each other. For this reason, the degree of freedom of lens design is limited, and thereby optical performance such as an aberration correction function is deteriorated. Since the spectacle lens disclosed in JP HEI 5-323185A is designed such that an inclination angle at a centration point is relatively small and a base curve is relatively shallow, the degree of wraparound becomes small and thereby the degree of covering the temporal region side of a head (i.e., a wrapping property) becomes insufficient. The term “inclination angle at a centration point” means an angle formed between an outward normal line normal to a front surface and an axis passing through a center of rotation of an eye and the centration point.
In Japanese Patent Provisional Publication No. P2000-506628A (hereafter, referred to as JP 2000-506628A), a spectacle lens configured such that off-axis aberrations are corrected by providing an aspherical surface on its front surface, and that off-axis aberrations are corrected by changing the degree of curve of a toric surface or a spherical surface provided on its back surface. However, since both of back and front surfaces of the spectacle lens disclosed in JP 2000-506628A are designed to be symmetric, it is impossible in principle to correct asymmetrical aberrations which increase as the inclination angle at the centration point becomes larger. The spectacle lens is configured to only achieve a balance between aberrations.